The present invention relates to fluid flow plates used in fuel cell stacks to distribute fluids such as fuel, oxidant and water over active surface areas of electrodes.
Conventional electrochemical fuel cells convert fuel and oxidant into electrical and thermal energy and a reaction product. A typical fuel cell comprises a membrane-electrode assembly (MEA) sandwiched between an anode flow field plate and a cathode flow field plate. Gas diffusion layers may be disposed between each flow field plate and the MEA to better distribute the fuel and oxidant to the MEA. Gaskets may be used to separate various layers and to provide requisite seals. The flow field plates typically include one or more channels extending over the surface of the plate adjacent to the MEA for delivery of fluid fuel or oxidant to the active surface of the MEA.
In a conventional fuel cell stack, a plurality of cells are stacked together, so that the anode flow field plate of one cell is adjacent to the cathode flow field plate of the next cell in the stack, and so on. In some arrangements, bipolar flow plates are used so that a single flow field plate has fluid flow channels in both sides of the plate. One side of the bipolar plate serves as an anode flow plate for a first cell and the other side of the flow plate serves as a cathode flow plate for the adjacent cell. Power can be extracted from the stack by electrical connections made to the first and last flow plate in the stack. A typical stack may comprise only a few or many tens or even hundreds of cells. The present invention is relevant to all of these various fuel cell stack constructions.
The process of forming fluid flow channels in the surfaces of the anode and/or cathode flow field plates can be achieved in a number of ways, such as by etching away, or otherwise removing, material from the surfaces of the plate in the desired channel pattern. This process is generally only suitable for plates that are thick enough to enable material removal. Another approach is to form the flow plates as corrugated plates with peaks and troughs defining the channels. This process is suitable for the formation of flow plates from very thin material.
Corrugated plates can be formed by a number of known processes including roll forming, hydroforming and pressing flat sheets using a punch and die. The present disclosure is particularly related to forming pressed plates using a pressing tool such as a punch and die. The punch and die both have surface relief features, which are used to impress corresponding features into the flat sheet.
A problem which exists in the formation of flow plates as pressed plates is that dimensional errors often arise when reproducing the surface relief features of the die into corresponding relief features in the pressed plate. For example, the flow channel widths and/or flow channel depths may vary from that intended. It is highly desirable to achieve dimensional accuracies in the flow channel features of a flow field plate of the order of ±10 microns.
It is an object of the present invention to reduce or eliminate dimensional errors in pressed flow field plates when reproducing relief features in the plates from a die.